1. Field of the Invention
The invention relates to an improved method for the production of 2,3,5-trimethylhydroquinone by rearranging 4-oxo-isophorone (keto-isophorone, 3,5,5-trimethylcyclohex-2-en[e]-1,4-dione) to a trimethylhydroquinone diester and by its subsequent saponification. ##STR2##
Trimethylhydroquinone is an important initial product for producing vitamin E.
2. The Prior Art
The rearrangement of keto-isophorone in the gaseous phase with a zeolite into trimethylhydroquinone is already known (DE 26 46 172 C2). However, the yields in this reaction are only small (50% at 30% conversion) and thus unsatisfactory for an economical method. In another method (Y. A. Joe, Y. M. Goo, Y. Y. Lee, Bull. Korean Chem. Soc. 1991, 12, 253) the rearrangement is carried out in 5% solution in acetic anhydride by the addition of five equivalents of concentrated sulfuric acid. Trimethylhydroquinone esters are obtained thereby with only 31% yield, so that even this method is not economical.
According to a third method (DE-OS 2 149 159) keto-isophorone can be reacted in the presence of a protonic acid in acetic anhydride to trimethylhydroquinone diacetate, which is subsequently saponified to 2,3,5-trimethylhydroquinone. Disadvantages in this method are the use
Of large amounts of acetic anhydride (5-10 mole/mole ketoisophorone), PA1 Of large amounts of the catalytic acid (up to 150 molar %) PA1 As well as the rather moderate yield with a maximum of 66%. PA1 The yields according to the method of the invention are up to 25% greater than in the cited literature and are between 85 and 90%. PA1 The amounts of catalyst required are approximately 0.1 to 50% compared to 150% in the literature. PA1 Only &gt;2 to 4 moles of a carboxylic acid anhydride per mole keto-isophorone are required compared to 5 to 10 moles in the literature. PA1 The saponification of the isolated trimethylhydroquinone ester with aqueous acid succeeds preferably in a simple manner in the presence of a phase agent.